Various chemical-analysis instruments utilize conduits, such as tubes, columns, and capillaries, for separation of compounds and/or delivery of fluids. For example, techniques such as liquid chromatography (LC), capillary electrophoresis (CE) and capillary electro-chromatography (CEC) separate compounds as the compounds pass through a conduit. In contrast, for example, an electrospray ionization (ESI) interface typically includes an emitter conduit through which a chromatographically separated eluent passes to create an ionized aerosol for mass-spectrometric analyses. through and/or exiting from the conduit. CEC, for example, utilizes an electro-osmotic flow (EOF) to propel a mobile phase through a chromatographic column. In contrast, high-performance liquid chromatography (HPLC) relies on an applied fluid pressure to propel a mobile phase through a chromatographic column.
For LC, CE, and CEC, for example, ESI is often used to provide an interface between a chromatographic conduit and a mass spectrometer. In an ESI interface, a voltage of approximately 3 keV to 4 keV is typically applied between an emitter capillary and a counter electrode spaced from an exit orifice of the emitter capillary. In contrast, for example, for CE and CEC instruments, a voltage is typically applied between electrodes located at either end of a conduit. To permit application of a voltage, a conduit is typically formed from a conductive material or includes conductive components.
In ESI, for example, a chromatographic eluent, including the analyte and solvent, is often forced by a carrier gas through a metallic capillary. A charge is transferred from the capillary to an aerosol of analyte and solvent sprayed from an exit orifice of the capillary. Typically, a neutral carrier gas is mixed with the aerosol to promote evaporation of the solvent and formation of separated ionized analyte molecules suited to mass-spectrometric analysis.
A variety of capillary diameters are used in emitters in ESI interfaces. For example, relatively narrow capillaries are often used for “nanospray” ESI to support, for example, smaller samples sizes and/or provide softer ionization of samples.
In some cases, instead of a metal capillary, an ESI emitter includes an insulating tube and a conductive electrode layer. For example, some ESI emitters are fabricated from non-conductive glass or fused-silica capillaries. The tips of such emitters are often made very small, e.g., a few micrometers in diameter, by pulling of the capillary tip. A voltage is applied to the emitter via, for example, a liquid contact to an electrode disposed away from the emitter tip, or, for example, via a gold coating on the non-conductive capillary. Such approaches, however, are often difficult to fabricate or control, and at times have less than desirable chemical and mechanical stability or durability.